#!/bin/sh
#
# Copyright (c) 2008 Silicon Graphics, Inc.  All Rights Reserved.
# 
# This program is free software; you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the
# Free Software Foundation; either version 2 of the License, or (at your
# option) any later version.
# 
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
# for more details.
#
# Replace a subtree in a performance metrics namespace (PMNS) with a new
# subtree read from a file.  The file is moved (note: not copied) into the
# PMNS and given the same name as the subtree.
#
# Usage: ReplacePmnsSubtree [-n namespace] pmns-subtree replacement-file
#
# Refer to the NOTE at the bottom of the file for important locking details.
#

. $PCP_DIR/etc/pcp.env

namespace=$PCP_VAR_DIR/pmns/root

# Print usage message to stderr and exit with status provided (default 1)

usageExit()
{
    echo "Usage: $prog [-n namespace] pmns-subtree replacement-file" 1>&2
    exit ${1:-1}
}

# Sanity check options, parameters, namespace, etc.

prog=`basename $0`
while getopts n:\? c
do
    case "$c" in
	n)
	    if [ "x$OPTARG" = x ]
	    then
		echo "$prog: -n requires a namespace" 1>&2
		exit 1
	    else
		namespace="$OPTARG"
	    fi
	    ;;
	\?)
	    usageExit 0
	    ;;
    esac
done

if [ ! -f $namespace ]
then
    echo "$prog: namespace doesn't exist: $namespace" 1>&2
    exit 1
fi

[ $# != 2 ] && usageExit

subtree=$1
newSubtreeFile=$2
namespaceDir=`dirname $namespace`

if [ ! -w $namespaceDir ]
then
    echo "$prog: can't write namespace directory $namespaceDir" 1>&2
    exit 1
fi

if [ ! -f $newSubtreeFile ]
then
    echo "$prog: can't read replacement namespace subtree file for $subtree:" \
	 "$newSubtreeFile" 1>&2
    exit 1
fi

# variables for back-up/restore of namespace files affected by this script

backups="root"
[ -f "$namespaceDir/$subtree" ] && backups="$backups $subtree"
backSuffix="$prog-$$-backup"
restore=false			# restore backup namespace files in cleanup()

# Signal and exit handler to clean/restore namespace (lock and backups).

haveLock=false

# signals we need to be careful of ... HUP, INT, QUIT, PIPE, ALRM, TERM
#
STD_SIGNALS="1 2 3 13 14 15"

cleanup()
{
    # Release namespace lock as early as possible.  Ignore signals to avoid
    # releasing a lock already released.
    trap "" $STD_SIGNALS

    if $restore
    then
	for f in $backups
	do
	    [ -f "$namespaceDir/.$f-$backSuffix" ] && \
		mv "$namespaceDir/.$f-$backSuffix" "$namespaceDir/$f"
	done
	$haveLock && unlockpmns $namespace
	haveLock=false
    else
	$haveLock && unlockpmns $namespace
	haveLock=false
	for f in $backups
	do
	    rm -f "$namespaceDir/.$f-$backSuffix"
	done
    fi
}

trap "cleanup; exit" 0 $STD_SIGNALS

# "haveLock=true" is duplicated in both "if" and "else" to minimise the window
# for a signal leaving an orphaned lock if the "else" condition fires (we
# stole an existing lock).  See note at bottom of script for details.

if lockpmns $namespace
then
    haveLock=true	# duplicate: minimise race condition (see note above)
else
    haveLock=true	# duplicate: minimise race condition (see note above)
    $PCP_BINADM_DIR/pmpost "PCP: PMNS lock stolen by: $*"
fi

# Namespace is locked, back up affected files.  Once backup completes, enable
# namespace restore during error handling.

backupErr=false
for f in $backups
do
    dest="$namespaceDir/.$f-$backSuffix"
    if cp "$namespaceDir/$f" "$dest"
    then
	:
    else
	echo "error creating namespace backup for $f" 1>&2
	backupErr=true
    fi
done
$backupErr && exit 1
restore=true

# pmnsdel leaves any file corresponding to the deleted subtree in place after
# it runs, regardless of whether it succeeds or fails.  Allow pmnsdel to fail
# (there may be no existing subtree).  Return 0 only if the entire replacement
# succeeds.

sts=0
pmnsdel -n $namespace $subtree >/dev/null 2>&1
if pmnsadd -n $namespace $newSubtreeFile >/dev/null 2>&1
then
    mv -f $newSubtreeFile $namespaceDir/$subtree
    sts=$?
else
    sts=1
fi
[ $sts = 0 ] && restore=false
exit $sts

##############################################################################

# NOTE
#
# If a signal occurs in the very short window between pmnslock returning and
# setting haveLock, the lock is not released by cleanup().  Any subsequent
# process calling pmnslock will block until its pmnslock steals the lock
# (currently after 120 secs).  That window is carefully minimised.
#
# Past versions of similar scripts (e.g. Rebuild) unconditionally unlocked the
# namespace when a signal was caught.  This could potentially happen while
# still waiting to acquire the lock, breaking a lock held by another process!
#
# Consider locking and backup/restore implications carefully if making changes.